The Environmental Protection Agency on Monday said that the widely used herbicide 2,4-D would remain on the market, denying a petition from an environmental group that sought to revoke the chemical’s approval.

The E.P.A. said that the environmental group, the Natural Resources Defense Council, had not adequately shown that 2,4-D would be harmful under the conditions in which it is used.

“At best, N.R.D.C. is asking E.P.A. to take a revised look at the toxicity of 2,4-D,” the E.P.A. said in its decision, which was posted on its Web site.

“Yet the ground for tolerance revocation is a lack of safety.”

First approved in the late 1940s, 2,4-D is one of the most widely used weed killers in the world. It is an ingredient of numerous home lawn-care products, and it is used by farmers.

Dow Chemical is thought to be the major manufacturer, though the E.P.A. has also approved versions from Nufarm, an Australian company, and Agro-Gor, a joint venture of PBI/Gordon of Kansas City, Mo., and Atanor of Argentina.

Use of the chemical is expected to grow substantially in the coming years because Dow is seeking federal approval to sell seeds of corn genetically engineered to be resistant to 2,4-D.

Farmers planting that corn would be able to spray 2,4-D on their fields to kill weeds without hurting the crop. Now, 2,4-D is not used much on corn, the nation’s most widely grown crop.

The council filed its petition in 2008 asking that the registration of the herbicide, as well as the permissible residue levels on various foods, be revoked. In February, it sued the E.P.A., saying the agency had not acted on the petition fast enough.

The group cited various studies suggesting that exposure to 2,4-D could cause cancer, hormone disruption, genetic mutations and neurotoxicity. It also said the E.P.A., in previous assessments, had underestimated how much people, especially children, might be exposed to the chemical through dust, breast milk and skin contact.

In its ruling, the E.P.A. said that while some studies cited suggested that high doses of the chemical could be harmful, they did not establish lack of safety, and in some cases they were contradicted by other studies.

The agency in particular cited a study, financed by the 2,4-D manufacturers and conducted by Dow, in which the chemical was put into the feed of rats. The study did not show reproductive problems in the rats or problems in their offspring that might be expected if 2,4-D were disrupting hormone activity, the E.P.A. said.

“E.P.A. has done a thorough job in evaluating all the evaluable data and found no cause for concern,” he said.

Mae Wu, a lawyer with the council, said the group was “disappointed that it has taken this long to deny our petition” and also “disappointed that they are not protecting public health by getting this toxic chemical off the market.” She said it was too soon to say what the group’s next step would be, though it will have the right to object to the ruling.

The E.P.A. has reviewed the safety of 2,4-D several times, particularly with regard to an increased risk of cancer.

Some studies have shown a higher risk of non-Hodgkin’s lymphoma among farmers who use the chemical. But E.P.A. reviewers have said the farmers might have been exposed to many things, making it difficult to state that 2,4-D was the cause.

After reviewing the data, the agency renewed the registration for 2,4-D in 2005. In 2007, it declined to conduct a special review of the cancer risk, saying that it had “determined that the existing data do not support a conclusion that links human cancer to 2,4-D exposure.”

2,4-D was an ingredient of Agent Orange, a defoliant used in the Vietnam War that is said to have harmed many Vietnamese civilians and American soldiers. Most experts say the main health problems came from contamination of 2,4,5-T, the other major ingredient in Agent Orange.

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